refactor(formula): extract formula parser into separate crate

Extract the formula AST and parser into a dedicated `improvise-formula`
crate and convert the project into a Cargo workspace.

The root crate now re-exports `improvise-formula` as `crate::formula` to
maintain backward compatibility for internal callers. The repository map is
updated to reflect the new crate structure.

Co-Authored-By: fiddlerwoaroof/git-smart-commit (gemma-4-31B-it-UD-Q4_K_XL.gguf)

Cargo.toml

@@ -1,3 +1,6 @@
+[workspace]
+members = [".", "crates/improvise-formula"]
+
 [package]
 name = "improvise"
 version = "0.1.0-rc2"
@@ -15,6 +18,7 @@ name = "improvise"
 path = "src/main.rs"
 
 [dependencies]
+improvise-formula = { path = "crates/improvise-formula" }
 ratatui = "0.30"
 crossterm = "0.29"
 serde = { version = "1", features = ["derive"] }